For a long time, fixed systems based on transmission X-ray imaging have been used for airport security. Over the last ten years or so, the requirements for security in public places have been growing and require systems on mobile platforms for detecting dangerous chemical substances or explosives concealed in baggage or parcels. The existing mobile systems make use in particular of X-ray backscattering. However, detection and identification capability remains limited. It is difficult in particular to discriminate between substances having similar densities. X-ray transmission is another technique that can be used. It provides access to a combination of the density of the material ρ and its effective atomic number Zeff, but not to each of these two quantities separately, and, in addition, the contributions from several elements constituting the package are superposed, this being dependent on the thickness traversed. 3D transmission imaging using a single energy enables the attenuation coefficient μ to be mapped at any point on an object. This technique therefore circumvents the traversed thickness problem.
The attenuation coefficient μ is a function of the density of the material ρ and of its Zeff, and depends on the energy. Multi-energy X-ray transmission in 3D finally enables ρ and Zeff to be determined.
There is a real need for systems providing reliable identification and rapid and easy implementation. These systems require the use of radiation sources that permit three-dimensional imaging without mechanical movement of the source system.
In most cases, the radiation sources use thermionic cathodes as electron emitters, but these solutions have several drawbacks:
In the case of directly-heated thermionic cathodes (FIG. 1A) having a filament Fil facing an anode A, or indirectly-heated thermionic cathodes (FIG. 1B) having a filament Fil heating an impregnated cathode Cath facing an anode A, a first limitation stems from the thermal inertia of such cathodes, preventing rapid modulation of the current and therefore of the X-ray dose rate (for a given energy, the dose rate is often controlled by the current output by the cathode; if the current rise or current fall is not steep, there will be transient X-radiation emission phases that may impair the quality of the received image on the detector). A second limitation stems from the need to have a complex power supply for the filament, if this is a high-voltage supply. The various insulating passages for biasing the grid, filament and cathode are also more complex and bulkier as they have to withstand the high voltages (20 to 600 kV) generally encountered in radiation-generating tubes.
To remedy the abovementioned problem of dynamically controlling the current, devices use a biased grid G, formed for example by wires or a mesh, or a pierced plate as illustrated in FIGS. 2A and 2B.
Thus, each radiation source generally consists of, as a minimum, a cathode, a filament and a current control grid (if the current is modulated), various high voltages being applied to them through a high-voltage insulator as shown in FIG. 2C. The final size of the radiation source is highly dependent on the dimensions of this insulator. Given these electrical connection and insulation constraints, it is very difficult to envisage two (or more) X-ray sources within the same vacuum envelope. Thus, the existing systems comprising several X-ray sources consist of several separate radiation-generating tubes.
In the case of field-emission cold cathodes emitting from tips, notably carbon nanotube tips, in the simplest version the filament and its power supply are omitted, as illustrated in FIG. 3A. However, this diode-type arrangement does not make it possible for the intensity of the emitted current to be controlled independently of the anode voltage. This is because the voltage is fixed by the desired X-ray energy, and the mechanical distance between the anode and the cathode is fixed, so that the electric field at the top of the nanotubes and the emitted current are also fixed. One advantageous arrangement as illustrated in FIG. 3B, consisting optionally of a focusing element F (electrostatic or magnetic focusing) and a biased extraction grid G, may allow the current to be controlled.
Among the main advantages of a cold cathode, notably one based on carbon nanotubes, over a conventional thermionic cathode are notably:                the elimination of a filament preheating time, resulting in immediate operational availability;        the absence of fatigue ageing due to the thermomechanical cycles encountered during start/stop sequences,        the elimination of the filament heated to high temperature and of the associated power supply, resulting in a reduction in the consumed energy and in a simpler power supply; and        the possibility of modulating the emission by biasing an extraction grid located in front of the carbon nanotube cathode.        
For a cold cathode, notably a carbon nanotube cathode, associated with a grid, there are however several limitations due to the presence of the grid in the field of application of radiation-generating tubes.
Among these limitations, the following may be noted:                the cathode-grid capacitance limits the maximum modulation frequency;        the current emitted by the cathode varies exponentially with the voltage applied to the grid, degrading the quality with which the current emitted by the cathode is controlled;        since the grid is not entirely transparent to the electron stream, it intercepts 30 to 50% of the current emitted by the cathode, promoting dimensional variations in this grid caused by it being heated, and consequently generating instability in the current emitted by the cathode because of the exponential variation mentioned above, while thermal inertia and embrittlement are aggravating factors;        the fraction of current intercepted by the grid and the grid heating resulting therefrom are also limitations for using this type of cathode with high currents (a few tens of mA). For example for a cathode of a radiation-generating tube with a voltage of 150 kV for a current of 2 mA, a grid intercepting 40% of the current would have to dissipate 120 W;        in the case of cathodes consisting of a plurality of tips, here nanotubes, a slight inhomogeneity in the geometrical characteristics of the tips results in a large distribution of the fields at the top and therefore in the currents emitted over the set of tips, with values possibly ranging from low emission up to destruction of the nanotube; and        it is also necessary to have a complex power supply for controlling the grid voltage relative to the high voltage.        
3D imaging devices are of two types. In the first type, the devices comprise an X-ray generator and, facing it, a detector for measuring the radiation that has passed through the object or the patient. To increase the number of viewing angles, these systems require the source and the detector, or the object or the patient to be rotated. These systems are generally unwieldy and complicated, and they require lengthy analysis times incompatible with the latest needs.
The second type permits 3D imaging techniques without any movement of the system or of the object. They require several X-radiation generators and several detectors for observation at various angles of incidence and requiring the images obtained to be recombined in order to extract 3D information therefrom. These “tomosynthesis” systems are simpler than those of the first type and may make it possible for the analysis times and the complexity of the system to be greatly reduced.
Finally, some radiation-generating tubes include, in addition to the high DC voltage, a linear accelerator (or “linac”) for bringing the electrons to very high energy so that they produce X-rays that are themselves of very high energy. Electrons are injected into the accelerating structure of a linear accelerator in its conventional configuration by means of an electron gun based on a thermionic cathode, with or without a grid. The electron emission is controlled by the cathode filament heating and/or the control grid bias.
Notably to meet the needs in X-ray medical imaging, the dose flux (Gy/s) must be controlled. Therefore, the emitted dose must be very stable, the dose depending on the uniformity of the electron current generated and on the quality of the device for regulating the photocathode current.